Note:In the spring of 1861, tensions between the northern and southern United States over issues including state's right versus federal authority, westward expansion and slavery exploded into the American Civil War - http://goo.gl/ETtxkC

Note:The First Transcontinental Railroad was a 1,907-mile contiguous railroad line constructed between 1863 and 1869 across the western United States to connect the Pacific coast with the existing Eastern U.S. rail network - http://goo.gl/Xafdg5

Note:Spanish Influenza killed over 500,000 people in the United States, and somewhere between 20 and 40 million in what has been cited as the most devastating epidemic in history - http://goo.gl/nYVCtq

Note:The United States enters World War II after the attack at Pearl Harbor, a global war that involved the vast majority of the world's nations and including all of the great powers - http://goo.gl/QfFDwG

Note:The Korean War was a war between the Republic of Korea (South Korea), supported by the United Nations, and the Democratic People's Republic of Korea (North Korea), at one time supported by the People's Republic of China and the Soviet Union - http://goo.gl/o1FrfM